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In the perspective of the forthcoming realization of the DORIS contribution to the ITRF2020, the IDS (International DORIS Service) Combination Center (CC) reviews the discrepancies between the estimated and physical tie vectors at the DORIS sites. From the ITRF2014 DORIS-to-DORIS tie discrepancies, we first discretize the tie differences with respect to its amplitudes with respect to the station position a-posteriori errors. Then, for the highest values of these two indicators, we look at different parameters (distance between the stations, time interval between the two stations, effect of the South Atlantic Anomaly - SAA, change in the site velocity, amplitude of the annual signals...) which may explain the discrepancies. When possible, we also look at the time evolution of the tie discrepancies between the ITRF2008 and ITRF2014 realizations as well as to the DORIS-to-GNSS tie differences. Finally, based on the latest DORIS cumulative position and velocity solution from 1993.0 to 2019.0, we forecast the ITRF2020 DORIS-to-DORIS tie discrepancies and the effect of longer time series and of the latest technical evolutions (e.g. new type of antenna, SAA corrections) on it. [ABSTRACT FROM AUTHOR]

We analyzed X-band radio tracking observations of the Mars Odyssey spacecraft during its aerobraking phase (October 2001-January 2002). Using the precision orbit determination software GEODYN, we obtained estimates of the spacecraft orbital energy lost during each periapsis pass due to atmospheric drag. We also recovered atmospheric density values at each periapsis, assuming simple exponential atmospheric models. Our measurements are in good agreement with the time series from the Odyssey accelerometer instrument, but they are dependent on the a priori scale height used. Using the accelerometer-derived periapsis densities and the precision orbit determination-derived frictional loss of orbital energy, we calculated new scale heights. Each represents the effective scale height of the atmosphere near periapsis for each aerobraking pass. Our results are consistently ≈1.7 ± 0.7 km greater than the published accelerometer values. The accelerometer measurements have higher spatial and temporal resolution when they are available, but these results provide a data set useful for engineering and navigational purposes, to assess variability in the Martian middle atmosphere. [ABSTRACT FROM AUTHOR]

Thermosphere densities from precise orbit determination of Mars Global Surveyor, Pioneer Venus Orbiter, and Magellan are used with contemporaneous data from six Earth-orbiting satellites to investigate the responses of these planetary satellite drag environments to changes in solar flux due to the sun's rotation. For comparative purposes, these results are cast in the form of equivalent exosphere temperature variations. Per 10-unit change in 10.7-cm radioflux (used as a proxy for extreme ultraviolet flux) reaching each planet, we find temperature changes of 20.6, 7.0, and 2.0 K for Earth, Mars, and Venus, respectively. The different responses are thought to reflect the differing efficiencies of CO2 cooling and extreme ultraviolet heating in these upper atmospheres, and thus provide an important constraint on planetary atmosphere models that seek to self-consistently and interconsistently simulate the thermospheres of these planets. Our results also provide new data for empirical density models that are used to predict the satellite drag environments of these planets. [ABSTRACT FROM AUTHOR]

Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately haft the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury's topography occurred after the earliest phases of the planet's geological history. [ABSTRACT FROM AUTHOR]

Abstract: A new analysis of the Doppler tracking data from the Lunar Prospector mission in 1999 revealed a number of previously-unseen gravity anomalies at spatial scales as small as 27km over the nearside. The tracking data at low altitudes (50km or below) were better analyzed to resolve the nearside features without dampening from a power law constraint, by partitioning the gravity parameters concentrated on either the nearside or farside. The resulting model presents gravity anomalies correlated with topography with a correlation coefficient of 0.7 or higher from degree 50 to 150, the widest bandwidth yet. The gravity-topography admittance of ∼70mGal/km is found from numerous craters of which diameters are 60km or less. In addition, the new model produces orbits that fit to independent radio tracking data from the Lunar Reconnaissance Orbiter and Kaguya (SELENE) better than previous gravity models. This high-resolution model can be of immediate use to geophysical analysis of small craters. Our technique could be applied to an upcoming mission, the Gravity Recovery And Interior Laboratory and useful to extract short wavelength signals from the MESSENGER Doppler data. [Copyright &y& Elsevier]

Abstract: DORIS is one of the four space-geodetic techniques participating in the Global Geodetic Observing System (GGOS), particularly to maintain and disseminate the Terrestrial Reference Frame as determined by International Earth rotation and Reference frame Service (IERS). A few years ago, under the umbrella of the International Association of Geodesy, a DORIS International Service (IDS) was created in order to foster international cooperation and to provide new scientific products. This paper addresses the organizational aspects of the IDS and presents some recent DORIS scientific results. It is for the first time that, in preparation of the ITRF2008, seven Analysis Centers (AC’s) contributed to derive long-term time series of DORIS stations positions. These solutions were then combined into a homogeneous time series IDS-2 for which a precision of less than 10mm was obtained. Orbit comparisons between the various AC’s showed an excellent agreement in the radial component, both for the SPOT satellites (e.g. 0.5–2.1cm RMS for SPOT-2) and Envisat (0.9–2.1cm RMS), using different software packages, models, corrections and analysis strategies. There is now a wide international participation within IDS that should lead to future improvements in DORIS analysis strategies and DORIS-derived geodetic products. [Copyright &y& Elsevier]

Describes the measurement and shape of asteroid 433 Eros from the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker Laser Rangefinder (NLR). Geodetic parameters from NLR190 shape, gravity and dynamics model; Description of a mesh view of the shape of Eros in the vicinity of Himeros.

Investigates the topography and gravity of Mars through data from the Mars Global Surveyor. Method used in calculating the Bouguer gravity; Interpretation of the Bouguer gravity; Major features of the free air gravity; Research findings.

Reports on observations from a global map of the topography of Mars from the Mars Orbiter Laser Altimeter. Dominant features, including the low northern hemisphere, the Tharsis province, and the Hellas impact basin; Details of the low northern depression; Three major drainage systems of the Mars topography; Volumes of the two polar caps.

Radio tracking of the MESSENGER spacecraft has provided a model of Mercury's gravity field. In the northern hemisphere, several large gravity anomalies, including candidate mass concentrations (mascons), exceed 100 milli-Galileos (mgal). Mercury's northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins. The low-degree gravity field, combined with planetary spin parameters, yields the moment of inertia CIMR² = 0.353 ± 0.017, where M and R are Mercury's mass and radius, and a ratio of the moment of inertia of Mercury's solid outer shell to that of the planet of Cm/C = 0.452 ± 0.035. A model for Mercury's radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron-sulfide layer and an iron-rich liquid outer core and perhaps a solid inner core. [ABSTRACT FROM AUTHOR]

A 3200-kilometers-long profile of Mercury by the Mercury Laser Altimeter on the MESSENGER spacecraft spans ~20% of the near-equatorial region of the planet. Topography along the profile is characterized by a 5.2-kilometer dynamic range and 930-meter root-mean-square roughness. At long wavelengths, topography slopes eastward by 0.02°, implying a variation of equatorial shape that is at least partially compensated. Sampled craters on Mercury are shallower than their counterparts on the Moon, at least in part the result of Mercury's higher gravity. Crater floors vary in roughness and slope, implying complex modification over a range of length scales. [ABSTRACT FROM AUTHOR]

Both poles of Mars are hidden beneath caps of layered ice. We calculated the density of the south polar layered deposits by combining the gravity field obtained from initial results of radio tracking of the Mars Reconnaissance Orbiter with existing surface topography from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft and basal topography from the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the Mars Express spacecraft. The results indicate a best-fit density of 1220 kilograms per cubic meter, which is consistent with water ice that has -15°/o admixed dust. The results demonstrate that the deposits are probably composed of relatively clean water ice and also refine the martian surface-water inventory. [ABSTRACT FROM AUTHOR]

The Orientale basin is the youngest and best-preserved major impact structure on the Moon. We used the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft to investigate the gravitational field of Orientale at 3- to 5-kilometer (km) horizontal resolution. A volume of at least (3.4 ± 0.2) × 10 6 km 3 of crustal material was removed and redistributed during basin formation. There is no preserved evidence of the transient crater that would reveal the basin's maximum volume, but its diameter may now be inferred to be between 320 and 460 km. The gravity field resolves distinctive structures of Orientale's three rings and suggests the presence of faults associated with the outer two that penetrate to the mantle. The crustal structure of Orientale provides constraints on the formation of multiring basins., (Copyright © 2016, American Association for the Advancement of Science.)

We have derived a gravity field solution in spherical harmonics to degree and order 900, GRGM900C, from the tracking data of the Gravity Recovery and Interior Laboratory (GRAIL) Primary (1 March to 29 May 2012) and Extended Missions (30 August to 14 December 2012). A power law constraint of 3.6 ×10 -4 / ℓ 2 was applied only for degree ℓ greater than 600. The model produces global correlations of gravity, and gravity predicted from lunar topography of ≥ 0.98 through degree 638. The model's degree strength varies from a minimum of 575-675 over the central nearside and farside to 900 over the polar regions. The model fits the Extended Mission Ka-Band Range Rate data through 17 November 2012 at 0.13 μm/s RMS, whereas the last month of Ka-Band Range-Rate data obtained from altitudes of 2-10 km fit at 0.98 μm/s RMS, indicating that there is still signal inherent in the tracking data beyond degree 900.

Spacecraft-to-spacecraft tracking observations from the Gravity Recovery and Interior Laboratory (GRAIL) have been used to construct a gravitational field of the Moon to spherical harmonic degree and order 420. The GRAIL field reveals features not previously resolved, including tectonic structures, volcanic landforms, basin rings, crater central peaks, and numerous simple craters. From degrees 80 through 300, over 98% of the gravitational signature is associated with topography, a result that reflects the preservation of crater relief in highly fractured crust. The remaining 2% represents fine details of subsurface structure not previously resolved. GRAIL elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies.

High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth.

The earliest history of the Moon is poorly preserved in the surface geologic record due to the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory (GRAIL) mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive-density anomalies that are interpreted to be ancient vertical tabular intrusions or dikes formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moon's radius by 0.6 to 4.9 kilometers early in lunar history, consistent with predictions of thermal models.

The responses of Earth's and Mars' thermospheres to the quasi-periodic (27-day) variation of solar flux due to solar rotation were measured contemporaneously, revealing that this response is twice as large for Earth as for Mars. Per typical 20-unit change in 10.7-centimeter radio flux (used as a proxy for extreme ultraviolet flux) reaching each planet, we found temperature changes of 42.0 +/- 8.0 kelvin and 19.2 +/- 3.6 kelvin for Earth and Mars, respectively. Existing data for Venus indicate values of 3.6 +/- 0.6 kelvin. Our observational result constrains comparative planetary thermosphere simulations and may help resolve existing uncertainties in thermal balance processes, particularly CO2 cooling.